[sdiy] SSM2164 VCF Problem

[Stewart Pye]

Hi,

I've breadboarded a 2164 based 4 pole VCF and it has a problem...

When the cutoff frequency is low (2164 output current low) there is a 
76Hz sine wave on the output which increases in level (but frequency 
stays the same) as the cutoff frequency is reduced. If there is a signal 
going through the filter is appears to wobble, but you can see the sine 
wave if there is no input to the filter.

I noticed this even when I first tried a single low pass section. It's 
just a 2164 into a TL072 integrator with  270pF cap, and a 30k feedback 
resistor back to the 2164 input.

Does anyone have any idea what's going on here?

Cheers,
Stew.